The present invention generally relates to ceramic capacitors, such as discoidal filter and chip capacitors. More particularly, the present invention relates to a hybrid capacitor having a design and arrangement of components which gives it both characteristics of a discoidal as well as a tubular capacitor.
Discoidal feedthrough capacitors are well known in the prior art and are used in a variety of electromagnetic interference (EMI) filtering applications. FIGS. 1 and 2 illustrate a typical EMI filtering discoidal feedthrough capacitor 10. The capacitor 10 includes a body 12 typically comprised of dielectric material. The capacitor 10 includes an outer metallization 14 and an inner metallization 16 lining a through hole, in this case a single center passageway through which a lead wire or pin 18 extends. The lead wire 18 is conductively coupled to the inner metallization 16, such as by means of conductive material 20.
In the prior art feedthrough capacitors, as illustrated in FIGS. 1 and 2, a plurality of active electrode plates 22 extend from the inner metallization 16, and a plurality of ground electrode plates 24 extend from the outer metallization 14, in spaced apart and stacked relationship so as to overlap each other. The one or more lead wires 18 passing through the one or more center holes of the capacitor 10 carry undesirable frequency signals. The capacitor 10 is a very efficient high frequency filter because of its extremely low inductance and broadband frequency attenuation characteristics.
However, a disadvantage of prior art discoidal feedthrough capacitors is that they are relatively costly to manufacture as compared to rectangular MLCC capacitors. Another disadvantage is that great care must be taken when the capacitors are in small sizes for high voltage applications, such as those for implantable cardioverter defibrillators. In medical implant applications, it is necessary to use gold brazes or other similar high temperature, biocompatible connections to make electrical and mechanical attachments to the capacitor and to conducting ground plane surfaces. Thus, it is very important, particularly in biomedical applications, that the capacitor be very strong in compression, sheer and torsion. The reason for this is that microfractures can occur which can lead to latent defects. For example, in a cardiac pacemaker, the shorting of such a capacitor could lead to the complete failure of the cardiac pacemaker and would, of course, be life-threatening. However, the capacitor must be as small and volumetrically efficient as possible as it is desirable to maintain the devices which are implanted as small as feasible.
With reference now to FIGS. 3 and 4, a prior art single wall tubular capacitor 26 is illustrated. With particular reference to the cross-sectional view of FIG. 4, one can see that there is an inside diameter metallization surface 28 lining the central passageway 30 of the capacitor 26. A lead wire or pin 32 extends through the passageway 32 and is conductively coupled, such as by material 34, to the inner metallization 28.
On the outer surface of the dielectric body 36 is an outer metallization 38. This is conductively coupled to a conducting ground plane surface 40, such as by an electrical and/or mechanical attachment 42. It is the overlap of the inside diameter metallization 28 and the outside diameter metallization 38 surfaces across dielectric material 36 that forms the feedthrough capacitance.
A significant disadvantage of such prior art tubular capacitors 26 is that if the wall thickness “t” is made sufficiently large enough so that the capacitor 26 is mechanically robust, then the capacitance drops significantly. It is a basic principle of all capacitors that as one increases the dielectric thickness there is an inverse relationship and the capacitance drops. However, if one were to make the dielectric thickness “t” very thin, one could achieve a relatively high capacitance value. However, the resulting structure would be very fragile and difficult to manufacture. This would also present drawbacks in medical implant applications where it is necessary to use gold brazes or other similar high temperature, biocompatible connections 42 to make electrical and mechanical attachments between the capacitor 26 and a conducting ground plane surface 40.
With reference now to FIG. 5, another tubular capacitor 44 is illustrated. The tubular capacitor 44 has been wound with active 46 and ground 48 electrodes within the dielectric material so as to be in an overlapping and parallel multi-layer configuration. An electrical lead wire or pin 52 extends through the passageway 54 of the capacitor 44, and is electrically attached 56 to an upper metallization 48, conductively coupled to the active electrode plates 46. A lower ground metallization 60 is conductively coupled to the ground electrode plates 48, and also conductively coupled, by material 62 to the ground plane structure 64.
While such multi-layer tubular capacitors 44 solve some of the structural problems otherwise associated with tubular capacitors, such as capacitor 26 illustrated in FIGS. 3 and 4, however, they are very expensive to manufacture and it is very difficult to maintain dimensional controls. Accordingly, the multi-layer structure illustrated in FIG. 5 is not commonly used.
U.S. Pat. No. 6,545,854 to Trinh et al. discloses a fringe-field, non-overlapping electrodes discoidal feedthrough ceramic filter capacitor. The capacitor 66 illustrated in FIGS. 6 and 7 is illustrative of the teachings of the '854 patent. The capacitor 66 includes an inner diameter metallization 68, and an outer metallization 70. An electrical lead wire or pin 72 extends through the passageway, and is conductively coupled to the inner diameter metallization 68. As disclosed in the '854 patent, there can be multiple passageways and electrical leads 72 passing through the capacitor 66. As illustrated in FIG. 7, there are a plurality of active electrodes 74 extending from the inner diameter metallization 68 and into the dielectric body 76. There are also a plurality of ground electrode plates 78 extending from the outer diameter metallization 70 and into the dielectric body 76. Trinh et al. teach, as illustrated in FIG. 7, that the plurality of ground and active electrode plates 74 and 78 do not overlap.
However, this capacitor 66 has been found to be very inefficient because the entire capacitance depends upon the fringe effects between the opposing electrode tips of the active electrode plates 74 and ground electrode plates 78. In addition, this is particularly disadvantageous for high voltage applications, in that, this creates what amounts to a tip-to-tip type of spark gap. It is a basic principle of high voltage engineering to avoid sharp points, particularly when you have two sharp points coming towards one another. Accordingly, if there were any kind of internal delamination, or what is known in the industry as a knit-line defect, between any of the opposed electrode ends 74 and 78, the high voltage field would rapidly propagate from tip-to-tip and short out the capacitor 66.
Accordingly, there is a continuing need for a mechanically robust, yet very small hybrid tubular discoidal capacitance structure, which has the characteristics of a discoidal feedthrough capacitor as well as a tubular capacitor. The present invention fulfills these needs, and provides other related advantages.